1. Field of the Invention
The present invention relates to a granular filler for a glass-ceramic material and in particular for a low melting solder glass for encapsulating semiconductors.
2. Description of the Prior Art
A filler of this kind is known from German Published Patent Application (DT-OS) 2,533,687. Many other materials, such as alkaline-earth halides, are also suitable for use as fillers.
By admixing a filler to a glass-ceramic material, the coefficient of expansion of the solder glass can be changed. The fillers given in German Published Patent Application 2,533,687, e.g. SiO.sub.2, have a low coefficient of expansion (SiO.sub.2 :5.multidot.10.sup.-7 /.degree.K.) and are therefore suited to reducing the coefficient of expansion of the solder glass.
When a glass-ceramic material is used, e.g., when a semiconductor device is encapsulated directly in a solder glass, it is important that the coefficients of expansion of the solder glass and the semiconductor substrate be approximately equal, with a difference between the coefficients of expansion of .+-.5.multidot.10.sup.-7 /.degree.K. being still permissible.
For example, when a gold-wire-bonded transistor alloyed onto a copper strip is used, the encapsulation of such a transistor must take place at temperatures below 378.degree. C., since above this temperature, gold and silicon form a eutectic soluble in the glass melt. A solder glass which permits encapsulation below 378.degree. C. is given below.
A disadvantage is that there is a great difference between the coefficients of expansion of this solder glass (100-120.multidot.10.sup.-7 /.degree.K.) and of the copper (160.multidot.10.sup.-7 /.degree.K.). This may result in the formation of cracks in the encapsulation. Therefore, either the coefficient of expansion of the solder glass must be adapted to that of the copper, or a different supporting material must be found to replace the copper. For the supporting material, alloys are known (e.g. an Fe-Ni-Co alloy with a high Fe content) whose coefficients of expansion agree relatively well with that of the solder glass. However, these alloys are much more expensive than copper and have a poor thermal conductivity.
As mentioned earlier, the coefficient of expansion of a solder glass can be changed by admixing a filler.